Often, the rocky cores of these planets are believed to be about the same size, while the distinguishing difference is the size of the atmosphere. Mini-Neptunes look more like gas giants, with a thicker atmosphere that would create too much pressure at the surface, and super-Earths have a much thinner layer.

A recent research study considered what would happen if a mini-Neptune migrated close to a low-mass star. M-class stars, as this type are known, have a volatile first billion years. The energy production from the stars can range drastically, with x-rays and extreme ultraviolet rays hitting planets with as much as 100 to 10,000 times more radiation than what the Earth experiences.

For habitability, this is a huge challenge. Because the star is smaller, rocky planets need to huddle in closer to be within the star’s habitable zone. The radiation emanating from the star in its youth slams into the atmosphere, stripping away molecules until there is little left.

What if, however, a mini-Neptune drifted in closer because the gravity of other stars or planets influenced its orbit? There appears to be a small set of situations where the planet could hold on to just enough atmosphere to be a “super-Earth,” a planet that is a little larger than Earth but still small enough to have a reasonable-sized atmosphere, according to new research led by Rodrigo Luger, a doctoral student in astronomy at the University of Washington in Seattle.

Artist’s impression of CoRoT-7b, a world 70% larger than Earth that orbits hellishly close to its parent star. It may have been larger in the past. Credit: ESO/L. Calçada

“It’s something that could potentially lead to the formation and evolution of life similar to what we know,” Luger said.

Luger’s team ran models of different kinds of planets and varied the eccentricity of their orbits, their masses and their diameters. The researchers discovered that a mini-Neptune would need to be no bigger than two or three Earth masses to transform into a potentially habitable super planet. If the planet is much bigger, its stronger gravity can hold on to most of the atmosphere, acting as a shield against the dwarf’s radiation but providing too much pressure at the surface.

But even if the mini-Neptune bleeds away enough atmosphere to become a super-Earth, it’s not clear if life could exist there. The model assumes a planet began with a hydrogen or helium atmosphere, which is common among gas giants. This sort of atmosphere is inhospitable to life as we know it.

Some scientists believe Earth’s atmosphere (which is mostly made up of nitrogen and oxygen) came by way of volcanic eruptions. Near a volatile M dwarf, however, the secondary atmosphere (atmosphere that was created after the planet was formed) could be stripped away on a newly created super-Earth just as quickly as the first, Luger pointed out.

Artist’s impression of a super-Earth, Kepler-62f. It’s unclear how much atmosphere is required to make a planet habitable. Credit: NASA Ames/JPL-Caltech

There are other problems as well. If the mini-Neptune was mostly made up of ice, close to the dwarf it could all melt after it migrated and form a water world. It’s unclear if life could form on a planet with no continents, which is key to generating the carbon cycle that fuels life on Earth. Further, the pressure of all that water could create high-pressure ice at the bottom of the ocean. This would prevent minerals from seeping out of the planet’s interior, which may also be necessary for life.

“The take-home message is these worlds are very different from Earth,” Luger said.

Observing challenges

Current technology can’t spot these worlds well yet, Luger said, as they are too far out from their parent star and too small and dim. If such planets were closer — too close to be habitable — they can be spotted through gravitational effects on the parent star, for example.

That’s what happened in the case of CoRoT-7b, a world that is 70 percent larger than Earth and orbits hellishly close to its parent star. When this planet’s discovery was announced in 2010, modeling showed it likely lost much of its atmosphere as it got closer to the dwarf star. This was found after looking at factors such as tidal forces, which happen when strong gravity from a star affects the rotation and orbit of a planet.

Luger hopes, however, that a new telescope will better spot these former mini-Neptunes. A NASA observatory called TESS (Transiting Exoplanet Survey Satellite) will be launched in 2017 and will be well-designed to look at planets orbiting dwarf stars, Luger said.

“It likely can detect planets in the habitable zones of M-class stars to see if they exist,” he said.

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